1. Field of the Invention
The present invention relates to acoustic transducers of the flexural strain gauge type capable of being submerged to a considerable depth without suffering damage, while still operating correctly. It applies to the transmission and/or reception of sonic or ultrasonic acoustic waves in fluid media such as the underwater space.
2. Discussion of the Background
Known flexural strain gauge transducers are generally made up of a leaktight flexible shell with a cylindrical side wall of elliptical cross-section, set vibrating by one or more columns or bars of ceramic piezoelectric cells. Each column is held in compression between the furthest apart opposing parts of the side wall. In transmission, an alternating electric field is applied in the longitudinal direction of each column and the resulting motion, which takes place along the longitudinal axis of each column, is retransmitted and amplified to the surrounding liquid medium, the amplitude of this motion being maximal in the plane generated by the minor axes of the ellipses formed by each cross-section.
A compressive prestress of the piezoelectric cells of each column is necessary in order to prevent the breakage of the ceramic when the columns are stressed in tension.
This prestress is, according to a first known embodiment, supplied directly by the shell when the columns are assembled. The housings provided in the shell for the columns have, before assembly, lengths which are shorter than those of the columns. In order to set the columns in place, it suffices to apply two opposing external forces to the closest together facing parts of the side wall in order to compress the shell at this location and cause, through elastic deformation of the shell, a just sufficient increase in the length of the housings to allow installation of the columns. The prestress force is applied when the action of the two external forces is removed. The columns then remain compressed in their housings between the parts of the internal side wall of the shell in contact with their ends.
In order to obtain correct operation of the transducers at a specified depth, this embodiment necessitates imparting to the amplitude of the two external forces a value greater than that which is normally exerted by the hydrostatic pressure at this depth. This has the disadvantage of limiting the use of these types of transducers to depths for which the prestress force of the column can still be ensured, in order to prevent the breakage of the ceramic making up the piezoelectric cells.
According to a second known embodiment, the prestress force of each column can be obtained by means of a rod passing through each column following its longitudinal axis, the ends of the rod being bolted to the shell. However in this case, the hydrostatic pressure exerts, via the shell, a tensile load on each column which, when it is too large, causes failure of the ceramic making up the piezoelectric cells.
Finally, according to a third known embodiment, a description of which can be found in the U.S. Pat. No. 4 420 826, the stack of piezoelectric cells can be produced along a prestress rod which is not fixed by its ends to the shell. Retention of the stack is ensured by two rails so as not to be subjected, as in the embodiment described earlier, to a tensile load directed along the longitudinal axis of the column. However, here again, when the submersion of the transducer is such that one or two sides of the columns are no longer in contact with the shell, the transducer can no longer operate correctly.
The Applicant has also proposed, in French Patent Application No. 88 14416 filed on Apr. 11, 1988, two other embodiments of a flexural strain gauge transducer in which a counterweight is added to the ceramic columns, which may possibly be provided by a fluidic device. These devices operate correctly but these additional units complicate their manufacture.